1. Field of the Invention
The present invention relates to a through-the-lens automatic focusing system, and more particularly to a control process for use in such system for driving a focusing lens in an interchangeable lens system.
2. Description of the Prior Art
There are already proposed various focusing control methods for automatic focusing devices for use in single-lens reflex cameras, and one of such methods relies on the detection of sharpness of an object image to control the focus position of an optical system in response to output signals from plural photoelectric converting elements for receiving the light passing through a focusing lens in an interchangeable lens. Such method is disclosed in the U.S. patent application Ser. No. 150,958 filed on May 19, 1980, of which inventors include the present inventor. This application was abandoned in favor of continuation application Ser. No. 3,330,438 filed on Dec. 14, 1981, now U.S. Pat. No. 4,443,086.
Automatic focusing system utilizing the abovementioned method provides advantages of a higher focusing speed and an accurate focusing operation even for an object under a relatively dark condition, in comparison with other conventional methods.
The above-mentioned method, however, does not provide perfect solution to the problems related to the use of various interchangeable lenses. The interchangeable lenses include various lenses such as a standard lens containing a movable focusing lens and a fixed imaging lens, a zoom lens containing a movable focusing lens and a movable magnification-changing lens, a zoom lens including a macro-photographic mechanism for enabling close-up photographing, etc. In most of such lenses, a focusing lens is rendered axially movable by a helicoid or cam mechanism in a lens barrel for focusing onto an object.
In such various interchangeable lenses, the driving force required for the motor for axially driving the focusing lens varies, depending on the loads of the helicoid or cam mechanisms.
Consequently, in case of driving the focusing lens by a motor according to the above-mentioned focusing method, differences in the driving characteristics resulting from the differences in the above-mentioned loads of interchangeable lenses give rise to fluctations in the focusing accuracy and/or in the focusing time, even when the same motor driving signal calculated according to the above-mentioned method is supplied to the motor driving circuit.
Also the output signal from the motor driving circuit in the camera is generally weak and is often unable to smoothly move the focusing lens because of the fluctations in the load thereof, thus giving blurred image.